An ignition system is proposed in the prior art which produces spark discharges repeatedly, rather than a single spark discharge, during an ignition period in order to increase the probability of allowing a fuel to be ignited, and is disclosed, for example, in Japanese Laid-Open Patent Applications No. 58,430/1975 and No. 28,871/1982. The ignition system disclosed in Japanese Laid-Open Patent Application No. 58,430/1975 assures the ignition of the fuel by increasing the chance that a fuel can be ignited, essentially by continuously repeating spark discharges during an ignition period. However, the supply and the delivery of appropriate amount of ignition energy is not always assured if the ignition takes place at a given time interval, due to a fluctuation in the secondary current of an ignition coil which is attributable to discharge phenomena occurring at the spark electrodes, variations in the response of ignition plugs during the manufacturing process or a variation from internal combusion engine to engine.
In the ignition system disclosed in Japanese Laid-Open Patent Application No. 28,871/1982, both the primary and the secondary current of an ignition coil are detected, and the on/off timing of the primary current is controlled in a manner corresponding to the actual current flow through the secondary winding to assure a secondary current of a given magnitude, thereby achieving a stabilized multiple ignition. However, the on/off control of the primary current through the detection of the secondary current present difficulties in its practical use because of the voltage withstanding capability which is required in detecting the high voltage across the secondary circuit. In addition, in an arrangement which performs a multiple ignition, it is necessary to provide an increased pause interval between repeated spark discharges in order to secure a sufficient amount of discharge energy. In addition, disturbances may be caused in the waveform of the discharge current of respective single sparks depending on conditions which prevail within a combustion chamber, and accordingly even a multiple ignition (repeated spark discharges) is not sufficient to assure an ignition with high energy.